Ultra-wideband (UWB) is an impulse radio based positioning and communication technology applicable primarily for indoor applications that require very large area tracking. Attractive properties of UWB compared to other RF-based communication and positioning technologies include a high immunity to interference and multi-path effects, using very small amounts of energy. UWB is often used in asset tracking systems, e.g., in health-care, logistics or manufacturing. Commercially available systems typically consist of a network of synchronized UWB receivers which track a large number of small, battery powered and inexpensive UWB transmitters, typically capable of transmitting signal bursts at several times per second. One example of such known systems is PLUS from Time Domain of Huntsville, Ala. in the United States. Another is the Series 7000 System from Ubisense Ltd. of Chesterton, Cambridge, in the United Kingdom.
Reported indoor position accuracies lie in the order of several decimeters for horizontal plane tracking only (2D), under favorable conditions, however, UWB technology suffers from numerous potential error and failure mechanisms such as: signal blockage due to signal absorption or attenuation and signal lock on strong multi-path (reflections) instead of direct path signal (e.g. when it is attenuated) or transmission through a material with a refraction index larger than that of vacuum causing a delay in the measured time of flight of the signal. Often such errors occur under non-line-of-sight (NLOS) conditions, but the positions derived from UWB systems also often include many outliers because of the poor geometry of practical receiver placements or the existence of multiple solutions to the resulting equations and the presence of noise, quantization and time measurement errors.
The limitations of UWB systems based on time of arrival (TOA) for position tracking due to practical limitations in the geometry of the placement of the readers is very important. Due to the fact that buildings and rooms are in practice often longer and wider than they are high, the choice in UWB receiver placement is limited. Systems known in the art consequently are typically limited to tracking in 2D (horizontally), assuming a known height of the UWB receiver and UWB transmitter. Systems that do perform 3D tracking perform very badly in tracking vertical position. Even in rooms or spaces that are very high, where the UWB receiver placement can be configured more advantageous for the purpose of tracking in 3D, the solution is very limited since a person would typically move on or close to the ground. This means that the geometry of UWB receiver placement is still not symmetric, causing less than ideal vertical and time dilution of precision.
These problems are most prominent while tracking moving objects or persons and can give rise to distorted and/or “jumpy” calculated trajectories. Although the performance obtained using UWB for asset management is often sufficient for that application and errors could be reduced for example by assuming a motion model, many potential application areas have much more stringent performance requirements, including increased accuracy, 3D position tracking, tracking of 3D orientation, smooth 3D velocity and angular velocity tracking, and a very high robustness against errors. Thus, the current state of the art in UWB tracking does not provide sufficient quality of measurement and error resistance to allow robust application in many application areas. Although these problems can be mitigated to an extent by complicated and expensive UWB receiver placement, it is an object of the invention to solve these problems in a more cost effective manner, while making the system easy to use, install and maintain, keeping in mind the practical geometries for typical buildings or outdoor setups.